If a malicious party gains physical access to a computer, he can boot an alternate operating system from a compact disk and overwrite the portion of the hard drive at which passwords are stored. By overwriting a legitimate password with a non-legitimate one, the attacker can gain unauthorized access to the system. For example, in the case of Microsoft Windows®, a hacker can overwrite a specific registry entry at which a hash of the administrator's password is stored with the hash for a known password. After rebooting the original system, the hacker can use this known password to login as the administrator. Once logged-in as the administrator, the hacker has full access to the computer.
This vulnerability can be overcome by hardware encrypting the entire hard drive. However, this requires the appropriate hardware and software support, and incurs a significant performance penalty to all hard drive access.
It would be desirable to be able to prevent unauthorized access to a computer system through password overwrites without requiring encryption of the hard drive.